The present invention relates to a coil for an MRI apparatus which forms at least two loops, and particularly to a coil for an MRI apparatus which is capable of enhancing the coupling characteristics of the reception coil by having a reduced coupling capacitance at the crossing section of the loops.
An MRI apparatus have been designed to implement the imaging process by detecting with a reception coil a magnetic signal which is created by the nuclear magnetic resonance. FIG. 7 is a diagram showing a developed view of a conventional saddle-type reception coil. In FIG. 7, a coil 101 forms a pair of loop coils 201 and 202 on the right and left, and the loop coils 201 and 202 are connected in series. The loop coils 201 and 202 have conductor patterns 105 and 106 which form loop conductor patterns 107 and a pattern crossing section 111. Disposed between the conductor pattern 106 and conductor pattern 107 is a resonance capacitor C1, which is connected to a cable section 103 for leading out a signal received by the coil 101. A balance/unbalance converting circuit such as an impedance matching circuit and balun is provided between the resonance capacitor C1 and the cable section 103.
The conductor patterns 105 and 106 cross each other at the pattern crossing section 111. FIG. 8 is a diagram showing the detailed structure of the pattern crossing section 111. In FIG. 8, the conductor patterns 105 and 106 cross each other by being interposed by a glass-epoxy substrate 121 which is an insulator. The conductor patterns 105 and 106 cross each other at right angles in order to reduce their magnetic coupling.
Based on this structure, there exists at the pattern crossing section 111 a coupling capacitance C, which is expressed in terms of the crossing area S of the conductor patterns 105 and 106, the thickness d of the glass-epoxy substrate 121, and the dielectric constant xcex5 of the glass-epoxy substrate 121 as in the following formula (1).
C=xcex5S/dxe2x80x83xe2x80x83(1)
The conductor patterns 105 and 106 have a width D, and the formula (1) is reformed as in the following formula (2).
C=xcex5xc2x7(Dxc3x97D)/dxe2x80x83xe2x80x83(2)
The conductor patterns 105 and 106 have their width D set large in order to reduce the resistance component of the coil. Consequently, the crossing area S is large. The glass-epoxy substrate 121 has its thickness d set small due to the limited layout space and cost of the coil 101. On this account, the coupling capacitance C of the pattern crossing section 111 is nonnegligible with respect to the resonance capacitor C1.
FIG. 9 is a diagram showing an equivalent circuit of the coil 101. This equivalent circuit forms a parallel resonance circuit. The impedance characteristic of this equivalent circuit is represented by a resonance curve which has a large impedance value at the resonant frequency fc as shown in FIG. 10. Generally, a coil has its Q value expressed in terms of the inductance L of the coil, the resistance component r of the coil, and the resonant frequency xcfx89 as in the following formula (3).
Q=xcfx89L/r=fc/xcex94fxe2x80x83xe2x80x83(3)
By setting a 3-dB band width xcex94f of the peak value on the resonance curve of FIG. 10, the Q value is evaluated by the formula (3). The resonant frequency fc relates to xcfx89 as xcfx89=2xcfx80fc, and the S/N factor (signal to noise ratio), which is a crucial parameter indicative of the quality of the tomographic image produced by the MRI apparatus, relates to the Q value as in the following formula (4).
S/Nxe2x88x9d(Q)xe2x80x83xe2x80x83(4)
As described above, the resistance component r increases with the increase of the coupling capacitance C, which results in a decreased Q value as suggested by the formula (3). The smaller Q value of the coil deteriorates the S/N factor as suggested by the formula (4), which results in a degraded quality of tomographic image. Namely, an increase of coupling capacitance C of the pattern crossing section 111 reduces the Q value of the coil 101, which gives rise to a problem of a degraded quality of tomographic image.
Therefore, it is an object of the present invention to provide a coil for an MRI apparatus which is designed to reduce the coupling capacitance C of the pattern crossing section 111 based on a simple structure so that the Q value of the coil 101 does not decrease, thereby producing a high-quality tomographic image.
In order to achieve the above objective, the coil for an MRI apparatus according to the first aspect resides in a coil for an MRI apparatus which forms a plurality of loops and has an insulated crossing section, and is characterized by including a first conductor pattern which forms a first loop and has its one end at the crossing section branching into a set of a prescribed number of first partial conductor patterns, and a second conductor pattern which forms a second loop and has its one end at the crossing section branching into a set of the prescribed number of second partial conductor patterns, and is further characterized in that each confronting pair of the first and second partial conductor pattern sets cross each other by being insulated from each other at the crossing section, and the adjacent first partial conductor patterns and adjacent second partial conductor patterns have their ends beyond the crossing section each connected together to other ends of the second conductor pattern and first conductor pattern by conductors which are spaced out from the second partial conductor patterns and first partial conductor patterns, respectively, by a prescribed distance or more.
The coil for an MRI apparatus according to the first aspect is designed to reduce the coupling capacitance of the crossing section based on the structure in which each confronting pair of the first and second partial conductor patterns each formed in a prescribed number of branches cross each other by being insulated from each other at the crossing section, and the first and second partial conductor patterns each have their ends beyond the crossing section connected together by conductors which are spaced out from the second and first partial conductor patterns by a prescribed distance or more.
The coil for an MRI apparatus according to the second aspect resides in a coil for an MRI apparatus which forms a plurality of loops and has an insulated crossing section, and is characterized by including a first conductor pattern which forms a first loop and has its one end at the crossing section branching into first partial conductor patterns of two in number, and a second conductor pattern which forms a second loop and has its one end at the crossing section branching into second partial conductor patterns of two in number, and is further characterized in that each confronting pair of the first and second partial conductor patterns cross each other by being insulated from each other at the crossing section, and the first partial conductor patterns and second partial conductor patterns have their ends beyond the crossing section each connected together to other ends of the second conductor pattern and first conductor pattern by conductors which are spaced out from the second partial conductor patterns and first partial conductor patterns, respectively, by a prescribed distance or more.
The coil for an MRI apparatus according to the second aspect is designed to reduce the coupling capacitance of the crossing section based on the structure in which each confronting pair of the first and second partial conductor patterns each formed in two branches cross each other by being insulated from each other at the crossing section, and the first and second partial conductor patterns each have their ends beyond the crossing section connected together to another end of the second and first conductor patterns by conductors which are spaced out from the second and first partial conductor patterns by a prescribed distance or more.
The coil for an MRI apparatus according to the third is characterized in that the first and second partial conductor patterns cross each other at right angles.
The coil for an MRI apparatus according to the third aspect is designed to reduce the crossing area thereby to reduce the coupling capacitance of the crossing section based on the structure in which the first and second partial conductor patterns cross each other at right angles.
The coil for an MRI apparatus according to the fourth aspect is characterized in that the partial conductor patterns have a virtually equal width.
The coil for an MRI apparatus according to the fourth is designed to reduce the crossing area thereby to reduce the coupling capacitance of the crossing section based on the structure in which the partial conductor patterns have a virtually equal width.
Therefore, the coil for an MRI apparatus according to the first aspect is designed such that each confronting pair of the first and second partial conductor patterns each formed in a prescribed number of branches cross each other by being insulated from each other at the crossing section, and the first and second partial conductor patterns each have their ends beyond the crossing section connected together by conductors which are spaced out from the second and first partial conductor patterns by a prescribed distance or more, so that the coupling capacitance of crossing section decreases, whereby the coil can have a large Q value to produce a high-quality MRI tomographic image based on a high S/N factor.
The coil for an MRI apparatus according to the second aspect is designed such that each confronting pair of the first and second partial conductor patterns each formed in two branches cross each other by being insulated from each other at the crossing section, and the first and second partial conductor patterns each have their ends beyond the crossing section connected together to other ends of the second and first conductor patterns by conductors which are spaced out from the second and first partial conductor patterns by a prescribed distance or more, so that the coupling capacitance of the crossing section decreases, whereby the coil can have a large Q value to produce a high-quality MRI tomographic image based on a high S/N factor.
The coil for an MRI apparatus according to the third aspect is designed such that the first and second partial conductor patterns cross each other at right angles at the crossing section, so that the crossing area is decreased to reduce the coupling capacitance, whereby the coil can have a larger Q value to produce a high-quality MRI tomographic image based on a high S/N factor.
The coil for an MRI apparatus according to the fourth aspect is designed such that the partial conductor patterns have a virtually equal width at the crossing section, so that the crossing area is decreased to reduce the coupling capacitance, whereby the coil can have a larger Q value to produce a high-quality MRI tomographic image based on a high S/N factor.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.